原式 = 10^x = 10^-3 / 7880
10^-3 / 10^x = 7880
10^ -(3+x) = 7880
-(3+x) log 10 = log 7880
-(3+x) = log 7880
-x = log 7880 + 3
x = -log7880 - 3
= -6.8965